It is generally accepted in the art of smoke detection devices that a dual ionization chamber arrangement is most advantageous in detecting smoke particles in the air due to combustion and to readily compensate for gradual ambient changes. The dual ionization chamber is particularly advantageous compared to a single ionization chamber in series with a resister which cannot compensate for ambient changes.
It has been generally accepted, that in order to provide ionized molecules in the dual chambers, a separate and distinct source of radioactive material is positioned in each chamber to ionize the medium and supply the ions. Several hundred or thousands of sources of radioactive material are usually made by die cutting them from large sheets of radioactive material. It has been found that the variation in distribution of radioactive material on such sheets and hence the amount of radioactive material on each die cut source can vary from source to source by as much as .+-.15%. It can therefore be appreciated that in using two separate and distinct sources, there can be a considerable difference in current flow established in two essentially identical smoke detector ionization chambers. This creates a significant problem because this wide range of variation between amounts of radioactive material from source to source requires complex calibration equipment and modifications to the internal electronics to provide an electrically balanced dual ionization chamber arrangement which has a mid-point voltage on the intermediate electrode. The importance of establishing the same current flow in each of the chambers thereby setting a constant midpoint voltage in the dual ionization chamber arrangement, will be discussed in more detail with respect to this invention.
The dual ionization chamber arrangement according to this invention overcomes the above problems in an efficient, economically manufactured manner and readily lends itself to changing the amount of radioactive material in each chamber to achieve a mid-point voltage for chambers of differing electrical characteristics.